Foreship arrangement for a vessel of the displacement type

ABSTRACT

A foreship arrangement for a vessel of the displacement type, which foreship consists of the part of the ship in front of the vessel&#39;s midship mark ( 2 ) and which vessel has a transversely symmetrical hull form about its centre line (CL) and a substantially conventional bow form below its design water line (Tdwl), characterised in that the stem line ( 1 ) of the vessel turns substantially backwards in relation to the length direction of the ship from a transition point (B) at or immediately above the design water line (Tdwl).

The present invention relates to a new design of the foreship of avessel of the displacement type. More specifically, the inventionrelates to an arrangement as disclosed in the preamble of independentclaim 1.

Ever since the start of oil activities in the North Sea, all vesselsengaged in marine operations have been characterised by having theirwheelhouse and superstructure with equipment located immediately behindthe forecastle deck and extending right up to the collision bulkhead. Alarge loading deck or working deck has been located aft of thesuperstructure and wheelhouse. This arrangement is a legacy of the firstplatform supply vessels which were used on the British continental shelfin the 1950s and is still the dominant solution today.

When these vessels are not in operation or are waiting on the weather(backing), they will normally have their bow turned upwind and into thewaves.

The bow design has been a raked bow or straight stem line up to thedesign water line and above that a raked bow, or a solution including abulb and a raked bow where the bulb can be defined as a bulge in thehull, primarily below the water line, to improve the wave system andresistance.

Typical negative effects of the aforementioned conventional bow formsare that they reflect incoming waves to too great a degree (waveformation gives energy loss), they are heavily subjected to the slammingof waves against the ship side, they generate a great deal of spray andthey are subjected to the occurrence of green water on the deck in frontof the superstructure.

When this type of bow shape meets incoming waves, the submerged volume(later referred to as “facing volume”) increases very quickly, buoyancyincreases as quickly and the retardation force becomes very large. Theseeffects intensify with increasing wave height and increasing speed ofthe vessel into the wave direction.

Vessels with their superstructure and wheelhouse located on the weatherdeck immediately aft of the collision bulkhead and having a hull shapeas described above are highly vulnerable to heavy weather damage.

The maximum speed at sea for these vessels is determined primarily bythe water line length, entry angles of the foreship and the proportionof outwardly sloping frame lines in the bow, and by available thrustforces from the propeller(s). Typical maximum speed in calm water isabout 13-16 knots, and they will normally have a loss of speed of about3 to 5 knots in head sea. The speed loss in head sea is a direct resultof the energy loss due to reflected waves, which results in retardationof the vessel.

Both during headway, in particular in head sea, and duringbacking/waiting, the crew, who have their quarters forward, willexperience large accelerations and retardations. Such motions reduce thequality of their rest periods and have an adverse effect on theircapacity to work. Reduced resting time and difficult working conditionsincrease the risk of accidents and injuries.

To reduce or eliminate the aforementioned drawbacks of the prior art,there is provided, according to the present invention, an arrangement asdisclosed in the characterising clause of claim 1.

Advantageous embodiments of the invention are set forth in the dependentclaims.

The design of the foreship according to the invention is intended fordisplacement hulls in the speed range of up to about 24 knots and withthe forward bulkhead of the superstructure preferably arranged forwardof amidships. The new bow shape is primarily intended to be used invessels that are engaged in marine operations, such as constructionvessels, pipelaying vessels, platform supply vessels, anchor handlingvessels, diving ships, etc.

The foreship is designed transversely symmetrical about the centre line(CL) of the vessel. The frame lines of the hull increase in width fromthe base line (BL). The bottom is flat or has a deadrise and merges intothe bilge at a given bilge radius. From the bilge up to a given height,the frame lines are slightly outwardly sloping. At the level of theforecastle deck, the outwardly sloping line form is terminated and isrun on upwards as a curved line form back towards the centre line.

According to the invention, a new form of foreship has been developedwhich reduces or eliminates the negative effects that the commonly knownbow shapes have. The foreship is designed to have slender water lines sothat the submersion of the facing volume takes place over a considerableperiod of time, which means that the vessel cuts into the wave and thewave laps over the bow form and out to the side. Thus, the buoyancyforce is distributed over time and the retardation forces aresubstantially reduced. This solution reduces reflection of waves, iteliminates the slamming of waves against the vessel's sides and bottom,the speed loss in head sea is reduced by about one knot as compared withthe standard bow shapes, and the stem line has a profile which isdesigned to prevent the waves from reaching too high up. Pitch and heavemotions will be reduced due to improved distribution of internal volumeand more slender entry lines of the foreship.

The hull in the example is run/extended up to the weather deck. At theweather deck, the stem line is advantageously bent forwards to form abarrier in the form of a spray board to prevent sea spray from reachingthis deck. This means that an enclosed volume is formed that extends upto the weather deck. Waves are thus allowed to creep up to the weatherdeck in the case of the largest waves.

The new design of the foreship will result in the following positiveeffects:

-   -   Lower accelerations and retardations, which give higher average        speed at sea, thereby reducing power requirement and consumption        of fuel.    -   A reduction in the amount of or the elimination of green water        on deck.

Model tests carried out by Marintek/SINTEF in February 2005 verify thepositive effects of the new design of the foreship.

Apart from the advantages mentioned above, the new foreship design willresult in:

-   -   A lower probability of heavy weather damage to the foreship        because the reflection of waves is reduced.    -   The elimination of the possibility of heavy weather damage to        the front bulkhead in the superstructure.    -   Improved working environment on board with regard to        -   accelerations and retardations, thereby improving safety            during navigation and providing higher operability,            especially in head sea.        -   reduction of noise and vibrations because of gentle motions            and reduced slamming of the waves against the hull, thereby            increasing comfort and improving safety with a view to            efficient utilisation of the crew's resting and working            time.    -   Protection of mooring equipment that is usually located on the        forecastle deck.    -   Simpler and stronger construction of skin plates and stiffeners        due to a large proportion of double-curved area.    -   A reduction in loads on skin plates and stiffeners due to        elimination of flare.    -   Smooth foreship all the way up to the bridge deck, which results        in reduced danger of icing. All deck equipment that is normally        exposed to wind, weather and icing is protected.    -   Smooth foreship all the way up to bridge deck, which results in        simpler installation of de-icing equipment.

In the following, a non-limiting embodiment of the arrangement accordingto the invention is described in more detail with reference to theaccompanying drawings, wherein:

FIG. 1 is a side view of the profile of the foreship stem line.

FIG. 2 is a front view of an extract of frame lines for the foreship.

FIG. 3 shows the water line for the foreship.

FIG. 4 is a perspective view of a hull, principally from below, with theforeship designed according to the invention.

FIG. 5 is another perspective view of the basic shape of the hull inFIG. 4, principally from the side.

FIG. 6 is a further perspective view of the hull in FIG. 4, principallyfrom the side.

In the following description and in the claims, unless otherwisespecified, all disclosures of direction are explained on the basis thatthe vessel is in a three-dimensional coordinate system where thevessel's length direction, breadth direction and height directioncorrespond respectively to the x-axis, y-axis and z-axis of thecoordinate system, wherein the x-axis and the y-axis are oriented in thehorizontal plane whilst the z-axis is oriented in the vertical plane.Furthermore, the forward direction of the ship corresponds to thepositive x-direction.

The new foreship, shown from the midship mark 2 of the vessel, has aslender and distinctive bow shape. FIG. 1 shows the vessel's stem line1, which starts at the base line 3 at point A and then rises with anincreasing curvature whilst being drawn forwards in the length direction(x-direction) to a point B slightly above the design water line, Tdwl.From point B, the stem line 1 rises further, but now with a diminishingcurvature and backwards (in the negative x-direction) until it reachespoint C. At point C, the stem line is advantageously bent forwards andis terminated as the top of spray board 5.

The frame lines of the foreship are designed transversely symmetricalabout the centre line (CL) of the vessel. FIG. 2 shows the frame lines10, 20, 30, 40, 50 of the hull which start at points D1, D2, D3, D4 andD5 and run almost perpendicularly from CL and increase in breadth (they-direction) from CL. The frame lines 10, 20, 30, 40, 50 thenrespectively merge into the bilge G1, G2, G3, G4, G5 at a given bilgeradius. From the bilge and up to points E1, E2, E3, E4, E5, the framelines 10, 20, 30, 40, 50, respectively, are outwardly sloping, and inthe embodiment shown in this figure at angles a2=11 degrees, a3=19degrees, a4=38 degrees and a5=30 degrees relative to the centre line CL.At points E1, E2, E3, E4, E5, the outwardly sloping frame line form isterminated and is run on upwards as a curved frame line form, eitherback to the centre line CL at points F1, F2, F3 or further upwards in avery gentle curve towards the centre line CL to points F4 and F5. Fromthe figure, it can also be seen that the bottom of the vessel is flat atzero intersection 7.

FIG. 3 shows the water line/entry angle seen in the xy-plane(length/breadth direction), which for the embodiment illustrated in thisfigure is 20.3 degrees, and which advantageously is between about 16 and25 degrees relative to the centre line CL at the design water line Tdwlfor reduced or increased slenderness.

FIGS. 4, 5 and 6 show the foreship according to the illustratedembodiment of the invention in different perspectives, advantageouslyprovided with spray board 5.

The table below shows advantageous ratios between water lines,slenderness and hull height for different ranges of water line length,where the water line Lwl is given in metres and at a given draught Tdwl,and where the hull breadth Bwl is measured at the zero intersection andat a given draught Tdwl.

Lwl 60-90 90-120 120-150 150-180 180-210 210- Bwl/Tdwl 2 3 3.5 3.5 4 5Lwl/Bwl 3 4.5 5 5.5 6 10 Lwl/Tdwl 5 13 17 20 22 23 Htdwl/Bwl 0.5 0.8 0.70.55 0.45 5 Lwl/Htdwl 2 5.5 7.5 10.5 13.5 15

The use of the above-mentioned ratios for the given water line lengthranges Lwl results in more slender entry lines, increased water linelength and only slightly outwardly sloping hull or frame lines (smallflare).

The abbreviations, which are used in this application, and in particularin the above table, have the following definitions:

-   -   Tdwl: Draught (at the design water line)    -   Bwl: Breadth measured at a given draught Tdwl    -   Lwl: Water line length measured at a given draught Tdwl; in        other words total length of the submerged volume.    -   Htdwl: Hull height measured from Tdwl up to the top of the spray        board.

For the illustrated and described exemplary embodiment, it may bespecified that Tdwl=6 metres, Lwl=81.1 metres, Bwl=18.5 metres andHtdwl=14.8 metres. The spray board 5 advantageously has a verticalheight of 1 metre, so that the height measured from Tdwl to thetransition to the spray board is thus 13.8 metres.

1. A foreship arrangement for a vessel of the displacement type, whichforeship consists of the part of the ship in front of the vessel'smidship mark (2) and which vessel has a transversely symmetrical hullshape about its centre line (CL) and a substantially conventional bowform below its design water line (Tdwl), characterised in that the stemline (1) of the vessel turns substantially backwards in relation to theship length direction (negative x-direction) from a transition point (B)at or just above the design water line (Tdwl).
 2. The arrangementaccording to claim 1, characterised in that the stem line (1), startingfrom a lower point (A) at the vessel's base line (3) rises and has asubstantially increasing curvature in the forward direction of thevessel to the transition point (B), and that the stem line (1) from thepoint (B) continues to rise, but with a substantially diminishingcurvature and in the aftward direction of the vessel, optionally brokenby one or more straight portions, to an upper point (C).
 3. Thearrangement according to claim 2, characterised in that a spray board(5) extends out from the upper point (C), the stem line (1) being bentsharply forward at said point (C) and being terminated at the top of thespray board (5).
 4. The arrangement according to claim 1, characterisedin that the vessel's flare angles in the foreship, and above the designwater line (Tdwl) are in the range of 9-45 degrees relative to theheight direction of the vessel.
 5. The arrangement according to claim 1,characterised in that the vessel's stem angles between the transitionpoint (B) and the upper point (C) increase from 0 degrees at thetransition point (B) to 55 degrees at the upper point (C) relative tothe height direction of the vessel.
 6. The arrangement according toclaim 1, characterised in that the entry angle of the bow at the designwater line (Tdwl). and in a plane coincident with the horizontal plane(the xy plane) is in the range of 16-25 degrees relative to the centreline (CL).
 7. The arrangement according to claim 1, characterised inthat the frame lines (10, 20, 30, 40, 50) of the foreship are designedtransversely symmetrical about the centre line (CL), and startingrespectively from first points (D1, D2, D3, D4, D5) run almostperpendicularly from CL and increase in breadth (the y-direction) fromthe centre line (CL), whereafter the frame lines (10, 20, 30, 40, 50)respectively merge into the bilge (G1, G2, G3, G4, G5) at a given bilgeradius, from which bilge and up to second points (E1, E2, E3, E4, E5)the frame lines (10, 20, 30, 40, 50) are outwardly sloping, and at whichpoints (E1, E2, E3, E4, E5) the outwardly sloping frame line form isterminated and then runs respectively upwards as a curved frame lineform back to the centre line (CL) at third points (F1, F2, F3) andcontinues upwards with very gentle curvature towards the centre line CLto third points (F4, F5).
 8. The arrangement according to claim 3,characterised by the following ratios for a water line length (Lwl) inthe range of 60-90 metres: Bwl/Tdwl=2, Lwl/Bwl=3, Lwl/Tdwl=5,Htdwl/Bwl=0.5 and Lwl/Htdwl=2.
 9. The arrangement according to claim 3,characterised by the following ratios for a water line length (Lwl) inthe range of 90-120 metres: Bwl/Tdwl=3, Lwl/Bwl=4.5, Lwl/Tdwl=13,Htdwl/Bwl=0.8 and Lwl/Htdwl=5.5.
 10. The arrangement according to claim3, characterised by the following ratios for a water line length (Lwl)in the range of 120-150 metres: Bwl/Tdwl=3.5, Lwl/Bwl=5, Lwl/Tdwl=17,Htdwl/Bwl=0.7 and Lwl/Htdwl=7.5.
 11. The arrangement according to claim3, characterised by the following ratios for a water line length (Lwl)in the range of 150-180 metres: Bwl/Tdwl=3.5, Lwl/Bwl=5.5, Lwl/Tdwl=20,Htdwl/Bwl=0.55 and Lwl/Htdwl=10.5.
 12. The arrangement according toclaim 3, characterised by the following ratios for a water line length(Lwl) in the range of 180-210 metres: Bwl/Tdwl=4, Lwl/Bwl=6,Lwl/Tdwl=22, Htdwl/Bwl=0.45 and Lwl/Htdwl=13.5.
 13. The arrangementaccording to claim 3, characterised by the following ratios for a waterline length (Lwl) in the range of 210 metres and above: Bwl/Tdwl=5,Lwl/Bwl=10, Lwl/Tdwl=23, Htdwl/Bwl=5 and Lwl/Htdwl=15.
 14. (canceled)